First you need to make sure the photo that you want has been tagged to that person. Then, click on the circle to choose the photo you want from all the photos tagged to that person.
@William Frank Maedgen
Plus, further to what 'Anne' has already proffered ...
Here is a "Knowledge Article" in "Familysearch":
How do I [ add or ] change a portrait photo in Family Tree?
I hope this helps.